Reduction or elimination of pattern placement error in metrology measurements

ABSTRACT

Metrology methods and targets are provided for reducing or eliminating a difference between a device pattern position and a target pattern position while maintaining target printability, process compatibility and optical contrast—in both imaging and scatterometry metrology. Pattern placement discrepancies may be reduced by using sub-resolved assist features in the mask design which have a same periodicity (fine pitch) as the periodic structure and/or by calibrating the measurement results using PPE (pattern placement error) correction factors derived by applying learning procedures to specific calibration terms, in measurements and/or simulations. Metrology targets are disclosed with multiple periodic structures at the same layer (in addition to regular target structures), e.g., in one or two layers, which are used to calibrate and remove PPE, especially when related to asymmetric effects such as scanner aberrations, off-axis illumination and other error sources.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/576,045 filed on Oct. 23, 2017, which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Technical Field

The present invention relates to the field of metrology, and moreparticularly, to targets and methods for reducing pattern placementerror (PPE).

2. Discussion of Related Art

Metrology measurements utilize a range of targets for measuring variousmetrology metrics, such as the overlay between target layers. Examplesinclude Standard Box in Box and AIM (advanced imaging metrology targetshaving periodical gratings with large pitches, e.g. 1500-2500 nm)imaging OVL (overlay) targets, and SCOL (scatterometry overlay) targets(having smaller pitches, e.g., 500-800 nm)—all of which have typicalpitches that are much larger than the design rule pitch of the devices,and hence have printability issues and also introduce errors such as thePPE (pattern placement error).

ASC (Archer Self Calibration) algorithms provided in U.S. Pat. No.9,329,033, which is incorporated herein by reference in its entirety,are used to correct inaccuracy and PPE simultaneously. Device-liketargets are provided in U.S. patent application Ser. No. 14/820,917,which is incorporated herein by reference in its entirety, yet facechallenges with respect to the optical contrast they provide and theirprintability.

SUMMARY OF THE INVENTION

The following is a simplified summary providing an initial understandingof the invention. The summary does not necessarily identify key elementsnor limits the scope of the invention, but merely serves as anintroduction to the following description.

One aspect of the present invention provides a method of reducing adifference between a device pattern position and a target patternposition of at least one periodic structure in a corresponding metrologytarget, the method comprising adding, to a mask design of the at leastone periodic structure, sub-resolved assist features at a sameperiodicity as, and in continuation of, the at least one periodicstructure, wherein the sub-resolved assist features have a CD (criticaldimension) smaller than a corresponding printability threshold.

One aspect of the present invention provides a method of reducing PPEdiscrepancy between a semiconductor device and a corresponding metrologytargets, the method comprising: deriving, from multiple targetstructures and metrology signals measured therefrom, a PPE correctionrelated to asymmetric aberrations, by applying a learning procedure to acalibration term, and adjusting corresponding measurements by the PPEcorrection.

These, additional, and/or other aspects and/or advantages of the presentinvention are set forth in the detailed description which follows;possibly inferable from the detailed description; and/or learnable bypractice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention and to showhow the same may be carried into effect, reference will now be made,purely by way of example, to the accompanying drawings in which likenumerals designate corresponding elements or sections throughout.

In the accompanying drawings:

FIG. 1 is a high-level schematic illustration of using assist featuresin periodic structures for metrology targets on the lithography mask,according to some embodiments of the invention.

FIG. 2 presents simulation results for the dependency of the differencebetween the CD values of leftmost and rightmost printed target elementson the CD of assist features (SRAF), according to some embodiments ofthe invention.

FIG. 3 is a high-level schematic flowchart illustrating a method,according to some embodiments of the invention.

FIG. 4 is a high-level schematic illustration of an example for targetsprepared by optimizing sub-resolved features to minimize targetasymmetry, according to some embodiments of the invention.

FIG. 5 is a high-level schematic illustration of targets with multipleperiodic structures in the same layer, according to some embodiments ofthe invention.

FIG. 6 is a high-level schematic illustration of targets with multipleperiodic structures in two or more layers, according to some embodimentsof the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the present inventionare described. For purposes of explanation, specific configurations anddetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will also be apparent to one skilledin the art that the present invention may be practiced without thespecific details presented herein. Furthermore, well known features mayhave been omitted or simplified in order not to obscure the presentinvention. With specific reference to the drawings, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

Before at least one embodiment of the invention is explained in detail,it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is applicable to other embodiments that may bepracticed or carried out in various ways as well as to combinations ofthe disclosed embodiments. Also, it is to be understood that thephraseology and terminology employed herein are for the purpose ofdescription and should not be regarded as limiting.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing”, “computing”,“calculating”, “determining”, “enhancing”, “deriving” or the like, referto the action and/or processes of a computer or computing system, orsimilar electronic computing device, that manipulates and/or transformsdata represented as physical, such as electronic, quantities within thecomputing system's registers and/or memories into other data similarlyrepresented as physical quantities within the computing system'smemories, registers or other such information storage, transmission ordisplay devices.

Embodiments of the present invention provide efficient and economicalmethods and mechanism for enhancing metrology measurements accuracy andthereby provide improvements to the technological field of metrology.Target designs and algorithmic approaches are provided for reduction ofscanner aberration impact on overlay (OVL) measurement overcoming priorart disadvantage of prior art target designs which do not allow tosatisfy simultaneously the requirements of target printability, processcompatibility, measurability (optical contrast) and small patternplacement error (relative to device pattern placement). It is noted thatwhile the disclosure is aimed at optical illumination radiation, it maybe extended to applications in which the illumination radiation is atvery short wavelengths such as x ray. Advantageously, disclosedembodiments overcome the difficulty posed by the need to use speciallydesigned periodic “proxy” targets with a coarse pitch P_(c) as metrologytargets which are not the devices themselves (which have opticallyunresolved design rule pitches), the metrology targets having largecoarse pitches (typically above 400 nm (e.g., for scatterometry methods)900 nm (e.g., for imaging methods) or more, compared with device pitcheswhich are typically below 90 nm, but may extend to a few hundreds ofnanometers as well) being measured by imaging and/or scatterometrymetrology procedures.

Disclosed embodiments remove or reduce the difference between themeasured target pattern placement error (PPE), being measured as part ofthe overlay, and the device pattern placement—which may result from thenon-agreeing structural scales of the devices and the metrology targets,and may exceed one or few nm in the prior art. Either or both approachesof target configuration and measurement correction and/or calibrationare disclosed below, and may be combined to enhance the measurementaccuracy. Disclosed target designs allow a significant relative PPEreduction without compromising on target process compatibility andtarget measurability (contrast) and disclosed target designs andalgorithmic approaches allow on-the-fly PPE corrections. It is notedthat while referring below to the overlay as a non-limitingrepresentative metrology metrics, other metrology metrics may be used inthe disclosed methods as well. It is emphasized that disclosedapproaches are applicable to both imaging metrology (e.g., AIM targetsand analogous structures) and scatterometry metrology (e.g., SCOLtargets and analogous structures).

Advantageously, disclosed targets remove the CD variation from printedtargets, and reduce the PPE associated therewith, which was found to belinked to the scanner aberrations. This discovery helps avoid thesensitivity increase of PPE to scanner aberrations by an order ofmagnitude which is expressed in the CD variation. The finding that CDvariation provides a pure imaginary (phase) addition to amplitudes offirst diffraction order and, correspondingly, provides neither focusslopes nor any overlay error magnification, nor requires a specificchoice of recipe setup to ensure accurate overlay measurement conditionsis utilized to develop target designs and measurement methods avoidingthese factors to identify, isolate, measure and remove the relative PPEto reach a resolution of aberration amplitude measurement which isbetter than 1 nm.

Metrology methods and targets are provided for reducing or eliminatingdiscrepancies in pattern placement error (PPE) between devices andcorresponding metrology targets, while maintaining target printability,process compatibility and optical contrast in both imaging andscatterometry metrology. Differences between target PPE and device PPEmay be reduced by using sub-resolved assist features having a sameperiodicity as the target's periodic structure and/or by calibrating themeasurement results using PPE correction factors derived by applyinglearning procedures to specific calibration terms, in measurementsand/or simulations. Metrology targets are disclosed with multipleperiodic structures at the same layer (in addition to regular targetstructures), e.g., in one or two layers, which are used to calibrate andremove PPE, especially when related to asymmetric effects such asscanner aberrations, off-axis illumination and other error sources.

Certain embodiments reduce or even remove the part of the PPE caused byasymmetric scanner aberrations, to improve the total accuracy of theoverlay measurements, while retaining an acceptable measurementperformance by using assist features having the same periodicity as thesegmentation pitch; measuring with only second harmonic of the signal;and/or applying calibration schemes to the metrology measurements, asdisclosed in detail below. Various embodiments may be implemented in anymetrology platform to improve matching with CDSEM (critical dimensionscanning electron microscope) overlay measurements (which is the mainreference for accuracy of overlay measurement) and to provide overlayvalues which are more relevant to real devices.

In certain embodiments, disclosed methods and targets may be configuredto handle asymmetries caused by scanner aberration(s) (as discussed in anon-limiting example in FIG. 1), by off-axis illumination for EUV(extreme ultraviolet), etc., and possibly enhanced by other factors suchas symmetric scanner aberrations (e.g., focus) or micro-loading effectsduring etching.

FIG. 1 is a high-level schematic illustration of using assist features112 in periodic structures 110 for metrology targets on the lithographymask, according to some embodiments of the invention. FIG. 1 provides aschematic comparison of target structures designs 90 on the mask havingperiodic structures 92 at a (fine) pitch P₁ which are all printed in thetarget with target structures designs 110 on the mask having periodicstructures 92 at a pitch P₁ which are printed, and assist features 112at a pitch P₂ (e.g., P₂=P₁) which are not printed; corresponding printedtargets 95, 115 with the printed elements having corresponding CDs(indicated CD₁, CD₂, CD₃, CD₄, etc.) and PPEs (indicated PPE₁, PPE₂,PPE₃, PPE₄, etc.) denoted 96, 116, respectively; and corresponding pupilimages 91, 111.

The inventors have found out that, in certain embodiments, designingtargets 110 to have P₂ close to P₁, or P₂=P₁, provides the benefitsdisclosed below. It is noted that disclosed methods and targets may beapplicable for both imaging and scatterometry metrology technologies,providing e.g., more uniform CD and smaller PPE in imaging metrology andmore uniform pupil diffraction signals as illustrated schematically inpatterns 95, 115 and 91, 111, respectively.

The inventors have found out a relation between asymmetric scanneraberrations and CD variability, as disclosed below (see Equation 1). Forexample, for the target elements illustrated in FIG. 1 (e.g., elementsfrom standard segmented AIM targets with pitch P_(c) between 1.2 and 2.4μm and coarse CD of ca.

$\left\{ {\sum_{n}{\Delta \; {{CD}_{n} \cdot {\sin\left( \frac{4\pi \; d_{n}}{P} \right)}}}} \right\}/{\left\{ {\sum_{n}{{\overset{\_}{CD}}_{n} \cdot {\cos\left( \frac{4\pi \; d_{n}}{P} \right)}}} \right\}.}$

illustrated without (90, 95, 91) and with (110, 115, 111) sub-resolvedassist features (SRAFs) 112 on the mask (110) having pitch P₂ which havethe same period as the segmentation pitch P₁. For target designs 90, CDvariability and PPE across pattern 95 are high (e.g., in the order ofmagnitude of several nm), while for disclosed target designs 110, CDvariability and PPE across pattern 115 are low (e.g., less than 1 nm, orless than 0.5 nm). The asymmetry of the aberrations may be expressed bythe CD difference between leftmost and rightmost lines of printedpatterns 95, 115, which may be e.g., few nm (e.g., ca. 2.5 nm) forprinted pattern 95 from target designs 90 lacking assist features; andless than 1 nm (e.g., less than ca. 0.5 nm) for pattern 115 from targetdesigns 110 with assist features 112.

FIG. 1 further presents the diffraction patterns 91, 111 in the pupilplane for respective target designs 90, 110 (the drawings represent onecoarse pitch of the target design). The two largest peaks in each of thediagrams correspond to the segmentation pitch (elements of the target'speriodic structures may be segmented by a fine pitch P₁, which is muchsmaller than the coarse pitch P_(c) and approaches the device pitch, inorder to enhance target printability) while all other smaller peakscorrespond to the coarse pitch P_(c). The largest diffraction peaks areresponsible for the shift of the respective target as a whole (the shiftis equal to the device shift due to aberrations when the segmentationpitch is equal or very close to the device pitch), while the otherdiffraction orders are responsible for CD modulation effects (e.g., thedifference between leftmost CD and rightmost CD). As illustrated inpattern 110, adding assist features 112 reduces the amplitudes of theother diffraction orders and therefore provides the assist featuresconfiguration which reduces the corresponding CD variability.

FIG. 2 presents simulation results for the dependency of the differencebetween the CD values of leftmost and rightmost printed target elementson the CD of assist features 112 (SRAF), according to some embodimentsof the invention. The simulation results show that as the size (e.g.,CD) of mask assist feature 112 increases, the difference betweenleftmost and rightmost CD's (asymmetric aberration) decreases, anddisappears when the SRAF size (of assist elements 112) approach the size(CD) of the regular line, elements 92, on the mask.

In the following analysis, the inventors have found out that the CDvariation provides the main impact on the discrepancy between positionof the device pattern and position of the overlay target (as it isobserved using the overlay measurement instrument). For example, ifthere were no CD variations when the target segmentation pitch P₁ ischosen to be equal to the device pitch, the overlay target as a wholewould be shifted exactly as the device due to the same scanneraberrations. Correspondingly, in this case there would be no discrepancybetween positions of device and the overlay target (no PPE discrepancy).

Equation 1 expresses a signal intensity I resulting from an interferencebetween only the zeroth and the +1^(st) diffraction orders, which arecaptured by an objective lens (for example on a CCD charge coupleddevice). A₀ and A₁ denote the amplitudes of the zeroth and the firstdiffraction orders respectively, Ψ denotes the phase difference betweenzeroth and first orders due to target topography and lens focusposition, GP denotes the grating position and the term

$R = \frac{F_{i}}{F_{i} - F_{j}}$

(scanner-aberration-induced term) describes the phase difference between+1 and −1 diffraction orders due to CD variations caused by scanneraberrations, where ΔCD_(n) is the CD difference of the n^(th) pair oflines symmetric with respect to the target center, CD _(n) is theaverage CD value and d_(n) corresponds to the distance of this pair oflines from the target center.

$\left\{ {\sum_{n}{\Delta \; {{CD}_{n} \cdot {\sin\left( \frac{2\pi \; d_{n}}{P} \right)}}}} \right\}/\left\{ {\sum_{n}{{\overset{\_}{CD}}_{n} \cdot {\cos\left( \frac{2\pi \; d_{n}}{P} \right)}}} \right\}$

As expressed in Equation 1, the scanner-aberration-induced term may beinterpreted as an additional shift to the true grating position anddetermines their PPE error due to the aberrations. For the designspresented in FIG. 1, the scanner-aberration-induced term provides anoverlay error of about 3.5 nm for target design 90 and about 1 nm fortarget design 110.

FIG. 3 is a high-level schematic flowchart illustrating a method 200,according to some embodiments of the invention. Method 200 may be atleast partially implemented by at least one computer processor, e.g., ina target design module and/or in a metrology module. Certain embodimentscomprise computer program products comprising a computer readablestorage medium having computer readable program embodied therewith andconfigured to carry out the relevant stages of method 200. Certainembodiments comprise target design files of respective targets designedby embodiments of method 200. Method 200 may comprise the followingstages, irrespective of their order, configured to achieve targetprintability, process compatibility, optical contrast and small patternplacement error, PPE and/or small or no PPE discrepancy between thedevices and the corresponding targets (stage 205).

Method 200 may comprise designing targets providing small or no PPEdiscrepancy, or relative PPE between the devices and the correspondingtargets (stage 210), according to any of the following embodiments.Method 200 may comprise comprising adding, to the mask design of targetperiodic structure(s), sub-resolved assist features at a sameperiodicity as, and in continuation of, the periodic structure(s) (stage212), to extend the target periodic structure with sub-resolved assistelements at the same pitch as the target's fine pitch and having a CD(critical dimension) smaller than a corresponding printability threshold(stage 215). Method 200 may comprise configuring assist features to havethe same periodicity as the target fine pitch (stage 220). Thedimensions of the SRAF assist features may be optimized to be thelargest which is still below the printability threshold (see e.g., FIG.2). The assist features may be configured e.g., by using the simulationapproach or by using a special mask with arrays of printed targets todetermine the printability threshold and limitations such as specialmasks with regular printability verification procedures, FEM (FocusExposure Matrix) wafers, CDSEM (critical dimension scanning electronmicroscope) etc. configured to determine the printability window.

In certain embodiments, method 200 may further comprise using optimizedphase shift masks to minimize target asymmetry (stage 230). The phaseshift masks may be optimized, e.g., using simulation(s) and/or empiricalmeasurements, to minimize the aberrations-related asymmetry of thetarget, while maintaining the total shift similar to the device shift.Using various methods, the assist features may be selected or configuredto minimize the CD variations, which together with selecting a finesegmentation pitch of the target elements which is equal to the devicepitch, minimizes or eliminates the relative PPE between the devices andthe corresponding targets. optimization may comprise e.g., OPC (opticalproximity correction)-like optimization with the free parameters of theoptimization including e.g., the number of mask features, theirplacement, each feature size and phase, etc.

In certain embodiments, method 200 may further comprise optimizingsub-resolved features to minimize target asymmetry (stage 235). Forexample, features on non-phase shift masks may be constructed usingperiodic structure(s) having sub-resolution pitch, which may actuallybehave similarly to phase shift masks, as derivable using the effectivemedium theory see e.g., the application of the effective medium theoryfor focus offset targets in U.S. Patent Application Publication No.2015/0309402, which is incorporated herein by reference in its entirety.

FIG. 4 is a high-level schematic illustration of an example for targets120 prepared by optimizing sub-resolved features to minimize targetasymmetry 235, according to some embodiments of the invention. Targets120 may be produced using an initial periodic structure 122, which maybe device-based and have a minimal design rule pitch (e.g., between80-150 nm, 30-100 nm or 10-50 nm, depending on the scanner technology)and applying a cutting procedure, illustrated schematically as cuttingmask 124, to yield overlay target 120 for the process layer, in whichtarget 120 has the smallest possible NZO (non-zero offsets), has noprintability problems as the target structures are similar in dimensionsto device structures and as there are no gaps in the pattern (e.g., nogaps larger than 100 nm) making targets 120 process-compatible. Astargets 120 also provide enough contrast for measurement optical tool,they are also adequate metrology targets.

In certain embodiments, method 200 may further comprise designing 1:1Line:Space (L:S) ratio of the target (or approximately so, e.g.,0.9<L:S<1.1) and measuring the second harmonics (the interference ofsecond and zeroth diffraction order signals without first orders) (stage240). Applying similar considerations as for Equation 1, thescanner-aberration-induced term, which describes the phase differencebetween +2 and −2 diffraction orders due to CD variations caused byscanner aberrations, is proportional to

$\begin{matrix}{I = {{A_{0} + {A_{1} \cdot \left\lbrack {e^{{i\; \psi} + {i\; \frac{2\pi}{P}{GP}} + {i\; \frac{\sum\limits_{n}{\Delta \; {{CD}_{n} \cdot {\sin {(\frac{2\pi \; d_{n}}{P})}}}}}{2{\sum\limits_{n}{C\; {{\overset{\_}{D}}_{n} \cdot {\cos {(\frac{2\pi \; d_{n}}{P})}}}}}}}} + e^{{i\; \psi} - {i\; \frac{2\pi}{P}{GP}} - {i\; \frac{\sum\limits_{n}{\Delta \; {{CD}_{n} \cdot {\sin {(\frac{2\pi \; d_{n}}{P})}}}}}{2{\sum_{n}{C\; {{\overset{\_}{D}}_{n} \cdot {\cos {(\frac{2\pi \; d_{n}}{P})}}}}}}}}} \right\rbrack}}}^{2}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The inventors conclude that, if the line:space ratio (L:S) is close to1:1, the extreme features influence on the measured overlay is close tozero, as then 4d_(n)≈P, and this part of the signal does not contain theinaccuracy related to scanner aberrations. Therefore, metrology targetsmay be designed and/or selected to have a 1:1 L:S ratio of the targetand so that the second harmonics include mainly the zeroth and secondorders interference (that is the −1^(st) and +1^(st) orders interferencepart in the second harmonics is much smaller). Accordingly, themetrology measurements of such targets are carried out with respect tothe second harmonics, and the expected inaccuracy (due to the extremefeatures difference) is reduced or removed (e.g., is close to, or equalszero).

Applying similar considerations to scatterometry overlay (SCOL)metrology, SCOL targets may be designed and/or selected to use seconddiffraction order signals instead of the first diffraction order signalsto minimize the impact of asymmetries such as ones related to scanneraberrations.

Method 200 may comprise correcting and/or calibrating measured overlayvalues to reduce PPE discrepancy and/or PPE value (stage 250) to removethe asymmetric component from the measurements. The following providesexample(s) for the calibration procedures. Targets comprise periodicstructures having well-established first and second harmonics (or anyother pair of harmonics) and are produced on at least two wafers withdifferent known aberrations A¹ and A². Metrology measurements mayinclude measuring the overlay using first harmonics only to yield OVL₁and measuring the overlay using second harmonics only to yield OVL₂ onthe at least two wafers. The harmonics may be measured using differentsetups (with respect e.g., to spectrum, focus, etc.).

Equation 2 expresses the difference between the overlays on differentwafers, assuming that it is due to scanner aberrations only, withPPE_(i) denoting the PPE of the i^(th) harmonic and RealOffset denotingthe real offset.

DiffOverlay_(i) _(th) _(harmonics) ^(wafer1,wafer2)=RealOffset+PPE_(i)==RealOffset(A ¹ −A ²)+F _(i)(A ¹ −A ²)  Equation 2

In the calibration process, if RealOffset is small (e.g., uponverification using simulation), it may be neglected, otherwise,RealOffset may be learned using the lithography simulation (e.g., eventhe aerial image simulation of dense periodic structure with a pitch andCD designed according to the fine pitch and fine CD of the target—maysuffice for the learning). Alternatively or complementarily, method 200may comprise learning PPE_(i) using, e.g., measurement simulation,according to Equation 2A—to yield the sensitivity of the measurement tothe asymmetry caused by the scanner asymmetric aberrations, which may belearned using lithography simulations. F_(i) denotes the slope of themeasurement error of the i^(th) harmonic due to aberration and isestimated empirically.

PPE _(i) =F _(i)(A ¹ A ²)=DiffOverlay_(i) _(th) _(harmonics)^(wafer1,wafer2)−RealOffset(A ¹ −A ²)   Equation 2A

From Equation 2A, the calibration constant

$R_{i,j} = \frac{F_{i}}{F_{i} - F_{j}}$

may be learned in a training phase, with respect one or more parameterssuch as the harmonic, the wavelength, the grating etc. During runtime,when measuring a new site and/or wafer having an unknown aberration Aexpressed as

$\left. \frac{P_{c}}{2} \right),$

with D expressed in Equation 3 and providing the corrected overlayexpressed in Equation 4.

D=OVL _(i) −OVL _(j) =PPE _(i) −PPE _(j)=(F _(i) −F _(f))A  Equation 3

OVL=OVL _(i) −PPE _(i) =OVL _(i) −R·D  Equation 4

In certain embodiments, the calibration presented above may be carriedout using different wavelengths instead of, or in addition to harmonics.

Applying similar considerations to SCOL metrology, SCOL targets may beconstructed to provide the second order and first order diffractionsignals, and SCOL measurements may be configured to provide at least twoseparate overlay measurements to which the above procedures may beimplemented, e.g., using any of different diffraction orders, differentwavelengths and/or different parts of pupil plane.

In certain embodiments, method 200 may further comprise removingasymmetric component(s) from the measured signals (stage 260).

In certain embodiments, method 200 may further comprise using multipletargets and metrology signals (such as diffraction orders forscatterometry measurements and harmonics signals for imagingmeasurements) to remove the aberrations (stage 270). Method 200 mayfurther comprise deriving, from targets and signals, a PPE correctionrelated to asymmetric aberrations, by applying a learning procedure to acalibration term (stage 272), with learning procedure being carried out,e.g., according to Equations 2-4, and possibly method 200 comprisesreiterating the derivation (stage 274) to calibrate the metrologymeasurements. In certain embodiments, method 200 may further compriseadjusting corresponding measurements by the PPE correction (stage 276).

In certain embodiments, method 200 may further comprise using targetswith multiple periodic structures in the same layer (stage 280), withthe periodic structures configured to provide the PPE correction uponmeasurement. The periodic structures may be identical or differ in pitchand/or CD. Method 200 may further comprise producing the multiple targetstructures in one layer by applying a cutting procedure to a periodicstructure at a minimal design rule pitch to yield periodic structures ofdifferent pitches and CD's (stage 290),

Embodiments of method 200 may be applied to scatterometry and/or imagingmetrology, depending on the specific configuration of targets andmeasurement procedures.

FIG. 5 is a high-level schematic illustration of targets 130 withmultiple periodic structures in the same layer (stage 280), according tosome embodiments of the invention. Certain embodiments comprise thefollowing calibration method and targets designed according thereto.

Targets 130 may comprise at least two periodic structures in at leastone same layer. For example, targets 130 may comprise a Triple AIM orequivalent target having three pairs of periodic structures in eachmeasurement direction, which is modified to have two of its gratingsprinted on the same layer (see e.g., FIG. 6 below). The periodicstructures in the same layer(s) may be similar or different by design,for example having different coarse pitches, and/or possibly, in somecases, different fine segmentation pitches (e.g., segmentation pitches96 nm and 160 nm), and distinctly different responses to the scanneraberrations. When similar, the periodic structures may have the samegeometrical asymmetry (e.g., have same fine CD and same fine Pitch) tomake the measurements independent of the type of asymmetric aberration,and therefore the asymmetric response invariant and the learnedconstants the same for all aberration and aberration-like PPEs. Thecoarse pitch and CD may be optimized using any of Equation 1, toolsimulation and/or empirically, e.g., to amplify the difference in themeasured overlay response between the periodic structures. Alternativelyor complementarily, the designs of the periodic structures may beconfigured to have proportional asymmetry which may possibly be verifiede.g., using simulation of lithography and/or process. In certainembodiments, the at least two periodic structures may be different inany of: the coarse CD, the coarse pitch, the span of the segmentation(some of which may be removed in one or more periodic structures) toamplify the difference in the measured overlay response between theperiodic structures as indicated by Equation 1. In certain embodiments,the targets may be produced and measured on at least two wafers, e.g.,having different known aberrations A¹ and A². Referring to FIG. 5,targets 130 may comprise two or more periodic structures 132A, 132B inthe same (at least one) layer, e.g., in a previous layer 132, andadditional periodic structure(s) 134 in one or more other layer(s),e.g., in a current layer 134. As discussed above, periodic structures132A, 132B may differ in any of their parameters (CD, pitch, extent anddimensions) or comprise similar structures.

In certain embodiments, the measurements of targets 130 having at leasttwo periodic structures in at least one same layer may be carried outaccording to the principles outlined above (Equations 2-4) and providedbelow. The overlay between two layers may be measured using grating F toprovide OVL₁ and using grating S to provide OVL₂ on both wafers (havingdifferent known aberrations A¹ and A²). The measurement may be carriedout using different setups (spectrum, focus, etc.) and using anyalgorithm. The processing may be carried out according to Equation 2,with PPE₁ denoting the PPE of the i^(th) grating and with RealOffsetneglected if small or learned using the lithography simulation and/orEquation 2A as disclosed above. The calibration constant

$A = \frac{D}{F_{i} - F_{j}}$

may be defined and learned using e.g., the first harmonics (ordiffraction orders in case of scatterometry measurements) to usemeasurements which are less sensitive to process variations than higherand/multiple harmonics (because the process variation changes the ratiobetween the amplitudes of different harmonics in the measured signal).Measuring some new site(s) and/or wafer(s) with unknown aberration A maybe carried out using Equations 3 and 4.

Applying similar considerations to SCOL measurements, two or moredifferent SCOL targets may be used, which have different target designsin one or more layer(s) and similar target designs in another one ormore layer(s), using similar algorithms and carrying out the calibrationaccording to the principles outlined above (Equations 2-4), with Rlearned using e.g., measurement tool simulation or using Equation 2.

In certain embodiments, calibration may be carried out by optimizing theillumination parameters such as spectrum and focus to achieve minimaltopographic phase for each harmonic, possibly for each site and/or foreach layer, as disclosed e.g., in U.S. Patent Application PublicationNo. 2017/0146915, which is incorporated herein by reference in itsentirety. The optimization may be carried out by scanning theillumination setup (e.g., spectrum, focus), on each site, and for eachperiodic structure (e.g., grating), in the vicinity of the periodicstructure and identifying the illumination setup (e.g., wavelength) withzero (or near zero) topographic phase for each harmonics; and thenmeasuring the corresponding periodic structure and the correspondingharmonics using the identified setup (e.g., wavelength and focusposition).

FIG. 6 is a high-level schematic illustration of targets 130 withmultiple periodic structures in two or more layers, according to someembodiments of the invention. Certain embodiments comprise quadruple AIMtargets 130, or equivalent targets, having four pairs of periodicstructures in each measurement direction, with possibly at least twoperiodic structures 132A, 132B and 134A, 134B in each of at least twolayers 132 and 134, respectively, such as the previous and currentlayers, respectively. Targets 130 may be used to measure and remove theasymmetric effects, such as scanner aberrations, from both previous andcurrent layers 134, 132, simultaneously. Certain embodiments comprisemultilayer targets 130 may be constructed (indicated schematically asoptional additional layers below previous layer 132), with multipleperiodic structures per two or more of the layers.

Aspects of the present invention are described above with reference toflowchart illustrations and/or portion diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each portion of the flowchartillustrations and/or portion diagrams, and combinations of portions inthe flowchart illustrations and/or portion diagrams, can be implementedby computer program instructions. These computer program instructionsmay be provided to a processor of a general-purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or portion diagram or portions thereof.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or portiondiagram or portions thereof.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/orportion diagram or portions thereof.

The aforementioned flowchart and diagrams illustrate the architecture,functionality, and operation of possible implementations of systems,methods and computer program products according to various embodimentsof the present invention. In this regard, each portion in the flowchartor portion diagrams may represent a module, segment, or portion of code,which comprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the portion mayoccur out of the order noted in the figures. For example, two portionsshown in succession may, in fact, be executed substantiallyconcurrently, or the portions may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each portion of the portion diagrams and/or flowchart illustration,and combinations of portions in the portion diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

In the above description, an embodiment is an example or implementationof the invention. The various appearances of “one embodiment”, “anembodiment”, “certain embodiments” or “some embodiments” do notnecessarily all refer to the same embodiments. Although various featuresof the invention may be described in the context of a single embodiment,the features may also be provided separately or in any suitablecombination. Conversely, although the invention may be described hereinin the context of separate embodiments for clarity, the invention mayalso be implemented in a single embodiment. Certain embodiments of theinvention may include features from different embodiments disclosedabove, and certain embodiments may incorporate elements from otherembodiments disclosed above. The disclosure of elements of the inventionin the context of a specific embodiment is not to be taken as limitingtheir use in the specific embodiment alone. Furthermore, it is to beunderstood that the invention can be carried out or practiced in variousways and that the invention can be implemented in certain embodimentsother than the ones outlined in the description above.

The invention is not limited to those diagrams or to the correspondingdescriptions. For example, flow need not move through each illustratedbox or state, or in exactly the same order as illustrated and described.Meanings of technical and scientific terms used herein are to becommonly understood as by one of ordinary skill in the art to which theinvention belongs, unless otherwise defined. While the invention hasbeen described with respect to a limited number of embodiments, theseshould not be construed as limitations on the scope of the invention,but rather as exemplifications of some of the preferred embodiments.Other possible variations, modifications, and applications are alsowithin the scope of the invention. Accordingly, the scope of theinvention should not be limited by what has thus far been described, butby the appended claims and their legal equivalents.

What is claimed is:
 1. A method of reducing a difference between adevice pattern position and a target pattern position of at least oneperiodic structure in a corresponding metrology target, the methodcomprising adding, to a mask design of the at least one periodicstructure, sub-resolved assist features at a same periodicity as, and incontinuation of, the at least one periodic structure, wherein thesub-resolved assist features have a critical dimension (CD) smaller thana corresponding printability threshold.
 2. The method of claim 1,further comprising using optimized phase shift masks to minimize targetasymmetry.
 3. The method of claim 2, wherein the phase shift masks areoptimized using simulation results of optical proximity corrections forthe metrology target.
 4. The method of claim 1, further comprisingoptimizing the sub-resolved assist features to minimize target asymmetryby deriving a maximal pitch therefor which is below a printabilitythreshold.
 5. The method of claim 4, wherein the optimizing is carriedout using simulations.
 6. The method of claim 1, further comprisingconfiguring the at least one periodic structure to have a Line:Space(L:S) ratio between 0.9 and 1.1, and measuring from the metrology targetsecond harmonics comprising an interference of second and zerothdiffraction order signals.
 7. The method of claim 1, applied to imagingmetrology and corresponding targets.
 8. The method of claim 1, appliedto scatterometry metrology and corresponding targets.
 9. The method ofclaim 7, wherein elements of the at least one periodic structure arefurther segmented at a fine pitch.
 10. The metrology target designedaccording to the method of claim
 1. 11. A method of reducing patternplacement error (PPE) discrepancy between a semiconductor device andcorresponding metrology targets, the method comprising: deriving, frommultiple target structures and metrology signals measured therefrom, aPPE correction related to asymmetric aberrations, by applying a learningprocedure to a calibration term, and adjusting correspondingmeasurements by the PPE correction.
 12. The method of claim 11, whereinthe learning procedure is carried out according toDiffOverlay_(i) _(th) _(harmonics) ^(wafer1,wafer2)=RealOffset+PPE_(i)=RealOffset(A ¹ −A ²)+F _(i)(A ¹ −A ²)  Equation 2,D=OVL _(i) −OVL _(j) =PPE _(i) −PPE _(j)=(F _(i) −F _(j))A,  Equation 3,andOVL=OVL _(i) −PPE _(i) =OVL _(i) −R·D,  Equation 4, wherein PPE_(i) is apattern placement error of an i^(th) harmonic, PPE_(j) is a patternplacement error of a j^(th) harmonic, A is an aberration of a wafer, A¹and A² are the aberrations of wafers 1 and 2, F_(i) is a slope of themeasurement error of the i^(th) harmonic, F_(j) is a slope of themeasurement error of the j^(th) harmonic, OVL is a corrected overlay,OVL_(i) is the overlay of the i^(th) harmonic, OVL_(j) is the overlay ofthe j^(th) harmonic, and R is a calibration constant.
 13. The method ofclaim 11, further comprising calibrating metrology measurements byreiterating the derivation of the PPE correction
 14. The method of claim11, further comprising configuring the multiple target structures in atleast one layer having two or more target structures to provide the PPEcorrection upon measurement.
 15. The method of claim 14, wherein the twoor more target structures in each layer have identical fine pitches. 16.The method of claim 14, wherein the two or more target structures differin pitch and/or critical dimension (CM.
 17. The method of claim 11,further comprising producing the multiple target structures in one layerby applying a cutting procedure to a periodic structure at a minimaldesign rule pitch to yield periodic structures of different pitches andCDs.
 18. The method of claim 11, applied to imaging metrology,corresponding targets and harmonics measured therefrom.
 19. The methodof claim 11, applied to scatterometry metrology, corresponding targetsand diffraction signals measured therefrom.
 20. Metrology measurementsderived from the method of claim 11.